The present invention relates to a DC-DC converter which supplies a stable output DC voltage in rapid response to a change in input/output conditions.
In recent years, as electronic equipment represented by mobile equipment has increased in functionality, the power consumption thereof has also increased. To use the battery of such electronic equipment for longer hours, the increase in the power consumption thereof should be suppressed. For this purpose, a technology termed power management that supplies power only to a currently operating circuit in electronic equipment and stops a circuit which need not be operated has been used commonly. A DC-DC converter which supplies a DC voltage to such electronic equipment is requested to rapidly respond to a change in output conditions.
For a DC-DC converter which is excellent in transient response characteristic, a known technology termed hysteresis control has been used conventionally. The hysteresis control is a self-excited control method which holds an output DC voltage within a hysteresis width set by a hysteresis comparison circuit. Since a feedback system using a typical error amplifier is not provided, a transient response time is dependent only on the delay time between the hysteresis comparison circuit and a driving circuit for a switch element. On the other hand, an output ripple voltage dependent on the equivalent series resistance (hereinafter referred to as ESR) of an output capacitor corresponds to a hysteresis width so that a switching frequency is directly proportional to the ESR. As a result, the problem is encountered that the use of an output capacitor with a low ESR lowers the switching frequency.
As a solution to the problem of the hysteresis control mentioned above, a technology as disclosed in, e.g., Patent Document (see Japanese Laid-Open Patent Publication No. 2004-64994) has been proposed. The technology superimposes a switching waveform on the waveform of an output DV voltage from a detection unit for the output DC voltage to increase the amplitude of the ripple voltage only in the detection unit, thereby preventing the lowering of the switching frequency and reducing the output ripple voltage. FIG. 6 is a view showing a circuit structure of a conventional DC-DC converter disclosed in Patent Document described above.
The conventional DC-DC converter shown in FIG. 6 includes: an input DC voltage source 10 for supplying an input DC voltage Vi; a switch element 11 and a diode 12 connected in series to each other in parallel relation to the input DC voltage source 10; an inductor 13 connected between the output of the DC-DC converter and the connecting point between the switch element 11 and the diode 12; an output capacitor 14 for supplying an output DC voltage Vo to a load 20; a resistor 15 and a capacitor 16 connected in series to each other in parallel relation to the inductor 13; and a hysteresis comparison circuit 18 for comparing a voltage Vx at the connecting point between the resistor 15 and the capacitor 16 with a reference voltage Vrf from a reference voltage source 17. The DC-DC converter uses an output from the hysteresis comparison circuit 18 to cause the ON/OFF operation of the switch element 11. The hysteresis comparison circuit 18 has a hysteresis voltage Vh and turns ON the switch element 11 when the voltage Vx at the connecting point between the resistor 15 and the capacitor 16 becomes not more than the reference voltage Vrf, while turning OFF the switch element 11 when the connecting-point voltage Vx becomes not less than a voltage (reference voltage Vrf+hysteresis voltage Vh).
A description will be given herein below to the operation of the DC-DC converter of FIG. 6.
First, when the switch element 11 is in the ON state, the voltage difference (Vi−Vo) between the input DC voltage Vi and the output DC voltage Vo is applied to the inductor 13 so that a linearly increasing current flows. The capacitor 16 is charged so that the voltage Vx at the connecting point Vx between the resistor 15 and the capacitor 16 rises. When the connecting-point voltage Vx eventually reaches the voltage (reference voltage Vrf+hysteresis voltage Vh), the hysteresis comparison circuit 18 turns OFF the switch elemenet 11. Then, when the switch element 11 is in the OFF state, the output DC voltage Vo is applied to the inductor 13 so that a linearly decreasing current flows. The capacitor 16 is discharged so that the voltage Vx at the connecting point between the resistor 15 and the capacitor 16 drops. When the connecting-point voltage Vx eventually reaches the reference voltage Vrf, the hysteresis comparison circuit 18 turns ON the switch elemenet 11. By repeating the operation described above, the voltage Vx at the connecting point between the resistor 15 and the capacitor 16 is controlled to upwardly and downwardly fluctuate between the reference voltage Vrf and the voltage (reference voltage Vrf and hysteresis voltage Vh).
Since the output DC voltage Vo is immediately transmitted to the hysteresis comparison circuit 18 via the capacitor 16, the transient response characteristic is excellent and the output ripple voltage can also be set to a low level.
In the DC-DC converter having such a structure, however, the output DC voltage Vo is not directly stabilized and a DC voltage superimposed on an output voltage from the capacitor 16 is present. This has caused the problem of degraded stability.